Determine whether photocoagulation therapy will be useful in preserving vision for patients with Diabetic Retinopathy. Assuming that therapy does have an effect, the study is designed to compare two commonly used treatment techniques (Xenon and Argon); determine whether treatment of selected areas of retina has a demonstrable morphologic effect on untreated areas; and if there are certain stages of the desease in which treatment is harmful; evaluate the efficacy of photocoagulation through simultaneous long-term wobservation of eyes not exposed to treatment.